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REMARKS ON COMPOSITION OF THE BLOOD. 
average diameter is said by Mr. Gulliver not to exceed the 
1-12325th of an inch. Gradation in their size in ruminants 
seems to prevail, the cells becoming smaller with the dimi¬ 
nution of the size of the animal. This, however, is so far 
from being the case among the mammalia in general, that it 
is ordinarily stated that the bulk of an animal has little to do 
with the size of the red cells of its blood. It would appear, 
ho wever, from recent investigations of Professor Quekett that 
the calibre of the capillary vessels of each individual animal 
is to be taken as a more correct standard of the size of the 
red cells than anything else—the two rising or falling to¬ 
gether. 
A great deal of discussion has taken place in former times, 
as to whether these cells were, or were not, nucleated in 
mammals ; different observers of equal eminence maintaining 
opinions the very opposite of each other. The matter is one 
of some importance, as elucidating the probable means of 
their reproduction. Like every other part of the organism, 
the red cells undergo changes which result in their ultimate 
dissolution or breaking up. If then they were nucleated, it 
is evident that they would follow the same law as all other 
cells of this class in their reproduction, namely , the setting at 
liberty of their nuclei by disintegration, which would then 
develop into new cells. 
The opinion that they possess a nucleus has doubtless had 
its origin in the circumstance that, when viewed as transparent 
objects, their bi- concave form gives them an appearance of 
having dark centres, from the refraction to which the rays of 
light are exposed. That this is the true cause of the phe¬ 
nomenon is proved by placing these bodies in a fluid less 
dense than that which they contain, when, by their imbibition 
of a portion of this, the dark spot disappears ; they being 
thus changed from bi-concave to flat-sided or even double- 
convex discs. This procedure would, on the supposition that 
they were nucleated, Lend, however, to bring the nucleus more 
into vision. The converse also is equally true, namely, the 
rendering the dark spot more distinct by emptying them of 
some of their contents, which is accomplished by placing them 
in a fluid of greater density than that which is located in their 
interior. 
It is a singular fact that, when considered in connexion with 
the blood-cells of other creatures, those of mammals should 
be enucleated. In birds, reptiles, and the amphibia, the red 
cells possess a nucleus; they are also very much larger than 
in the mammalia, a circuinstance which affords many ad¬ 
vantages for the study of their structure, &c., in these 
creatures. 
